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Microbial co-cultivation is a promising approach for the activation of biosynthetic gene clusters (BGCs) that remain transcriptionally silent under artificial culture conditions. As part of our project aiming at the discovery of marine-derived fungal agrochemicals, we previously used four phytopathogens as model competitors in the co-cultivation of 21 marine fungal strains. Based on comparative untargeted metabolomics analyses and anti-phytopathogenic activities of the co-cultures, we selected the co-culture of marine <i>Cosmospora</i> sp. with the phytopathogen <i>Magnaporthe oryzae</i> for in-depth chemical studies. UPLC-MS/MS-based molecular networking (MN) of the co-culture extract revealed an enhanced diversity of compounds in several molecular families, including isochromanones, specifically induced in the co-culture. Large scale co-cultivation of <i>Cosmospora</i> sp. and <i>M.</i> <i>oryzae</i> resulted in the isolation of five isochromanones from the whole co-culture extract, namely the known soudanones A, E, D (<b>1</b>-<b>3</b>) and their two new derivatives, soudanones H-I (<b>4</b>-<b>5</b>), the known isochromans, pseudoanguillosporins A and B (<b>6</b>, <b>7</b>), naphtho-<i>γ</i>-pyrones, cephalochromin and ustilaginoidin G (<b>8</b>, <b>9</b>), and ergosterol (<b>10</b>). Their structures were established by NMR, HR-ESIMS, FT-IR, electronic circular dichroism (ECD) spectroscopy, polarimetry ([α]_D), and Mosher’s ester reaction. Bioactivity assays revealed antimicrobial activity of compounds <b>2</b> and <b>3</b> against the phytopathogens <i>M. oryzae</i> and <i>Phytophthora infestans</i>, while pseudoanguillosporin A (<b>6</b>) showed the broadest and strongest anti-phytopathogenic activity against <i>Pseudomonas syringae</i>, <i>Xanthomonas campestris</i>, <i>M. oryzae</i> and <i>P. infestans</i>. This is the first study assessing the anti-phytopathogenic activities of soudanones.